1. Field of the Invention
The present invention relates to a circuit for generating a control voltage depending on a frequency and/or phase difference between a first periodic signal and a second periodic signal of arbitrary frequency where the first periodic signal produces a first trigger signal applied to a first input of a flip-flop for setting the flip-flop and where the second periodic signal produces a second trigger signal applied to a second input of the flip-flop for resetting the flip-flop.
2. Brief Description of the Background of the Invention Including Prior Art
Circuits set forth in the field of the invention are known as frequency and/or phase detectors, which in phase locked loops (PLL) provide the frequency or respectively phase comparison between a periodic input signal and the output signal of a voltage controlled oscillator serving as a reference signal. As an example for such frequency/phase detectors can be listed the integrated circuits MC 4344 or MC 4044 distributed by Motorola Corporation.
Such phase locked loop circuits are essentially noninductive, tunable, active filters with an adjustable band width. A phase locked loop in general comprises a voltage controlled oscillator, a phase detector and a loop filter. If the phase difference between the periodic signal from the voltage controlled oscillator and the input signal is constant, the phase loop is locked. If either the input or reference signal or the output of the voltage controlled oscillator change in phase, the phase detector and filter produce a D.C. error signal which is proportional in magnitude and polarity to the original phase change. This error voltage changes the voltage controlled oscillator phase so that it again locks into the reference signal. If a frequency divider is inserted in the feedback path of the phase locked loop between voltage controlled oscillator and phase detector then the output of the voltage controlled oscillator can be locked to some multiple of the reference frequency.
Frequency/phase detectors of the simplest kind have voltage/phase characteristic curves according to FIG. 2. The characteristic curves are periodic with a linear slope between the phase difference values n.multidot.2.pi. (n=0,.+-.1,.+-.2). Such phase detectors operate without problem once in the controlled state. However, difficulties occur in trying to reach the controlled state. In addition, there is always the possibility that at phase differences too large jumps by 2.pi. occur and the cycle slips.